The key senator behind a landmark congressional investigation into the CIA’s use of torture has rejected redactions made by the Obama administration ahead of a planned public release of the politically charged report.

In the latest struggle between senator Dianne Feinstein, the California Democrat who chairs the intelligence committee, and the CIA, Feinstein said she would delay a heavily anticipated disclosure of portions of the report in an attempt to reverse redactions that “eliminate or obscure key facts that support the report’s findings and conclusions”.

“Until these redactions are addressed to the committee’s satisfaction, the report will not be made public,” said Feinstein, who added that she intended to outline the committee’s desired disclosures in a private letter to President Barack Obama.

Another powerful senator and Obama ally, Carl Levin, the Michigan Democrat who chairs the armed services committee and who spearheaded his own investigation into US military torture, called the redactions “totally unacceptable.”

Feinstein said she had secured a commitment from the White House and the intelligence agencies “to working through these changes in good faith” – a position that would represent a shift for an intelligence community that only days ago portrayed the proposed redactions as minimal.

On Friday, after the White House provided the committee with a redacted version of the report for public release, director of national intelligence James Clapper issued a statement saying “more than 85% of the committee report has been declassified, and half of the redactions are in footnotes.” The White House put the CIA in charge of the redactions process, a move some observers considered a conflict of interest.

Clapper left the door open to a “constructive dialogue with the committee.” In an indication of the deep strains between the committee and the CIA, that dialogue is largely brokered by the White House, which is attempting to balance the competing interests of both powerful entities while each looks to Obama for support.

That balance has been harder to maintain after CIA director John Brennan on Thursday conceded that agency officials had been found to have violated a network firewall to access email and other data from committee staffers conducting the investigation. Brennan apologised, but several senators of both parties, on and off the committee, are calling for Brennan to resign or to be fired.

Feinstein, one of the most powerful legislators on intelligence matters, did not join the calls for Brennan’s head, although in March she accused the CIA of transgressing its constitutional boundaries.

Obama, who has for years counted Brennan amongst his most trusted aides and liaisons to the intelligence world, has expressed confidence in his CIA director. Yet on Friday he publicly conceded, “we tortured some folks,” using a verb that the CIA has resisted at every turn for describing its post-9/11 detentions, interrogations and renditions.

Despite Clapper’s statement, the committee has never proposed making more than a fraction of its inquiry public. It voted in April to declassify only the report’s findings, recommendations and executive summary, at most 700 pages of a report sprawling beyond 6,000 pages of narrative. The battle over the scope of the blackouts is waged only over those sections.

Yet for more than a year, senators on the committee have made clear that it has concluded the CIA materially misrepresented the scope, efficacy and intensity of its torture regime to both its legislative overseers and the Bush-era Justice Department. After at least one al-Qaida suspect, Abu Zubaydah, had been tortured, the department in 2002 lent its legal imprimatur to the practices on the basis of the CIA characterisations.

Republicans on the committee, including vice chairman Saxby Chambliss of Georgia, dissent from a conclusion that torture was an ineffective method of eliciting intelligence – the subject of years of internal and public debate, and even an Oscar-nominated Hollywood movie backed by the CIA.

Feinstein in March revealed that the committee would not have begun the investigation had not a CIA official, Jose Rodriguez, destroyed nearly 100 videotaped interrogations. She and others on the panel, after being initially assured that Rodriguez had not engaged in “destruction of evidence,” ultimately came to believe that the CIA was covering up activities that violated the law and contravened American values, a contention the CIA has rejected wholesale.

In an early July interview with the Wall Street Journal, Brennan - whose own position on torture while he was a top CIA official in the early 2000s is murky and the subject of both speculation and controversy - indicated that he would largely resist the Senate report’s conclusions.

“I will accept on behalf of the agency responsibility for failures, for problems and actions I believe should not have taken place. At the same time, I am going to take issue with some other elements of the report that I believe are inaccurate or misleading,” Brennan said.

Yet the agency’s credibility has sustained a major and self-inflicted blow over the past five days.

Having publicly rebuked Feinstein in March for accusing the CIA of spying on Senate staffers, Brennan on Thursday had to accept that conclusion. A report by the CIA”s own inspector general vindicated Feinstein and also found that employees of the agency were disingenuous when they made a counteraccusation of Senate violations of classified information. While the agency can count on the support of intelligence committee Republicans, it has few other allies in an anticipated campaign, which has reportedly drawn in several former agency directors, to discredit the Senate report publicly.

The Justice Department last month opted not to pursue any charges against either the CIA or the Senate intelligence committee, consistent with its historic reluctance to prosecute government officials involved in torture.

“Classification should be used to protect sources and methods or the disclosure of information which could compromise national security, not to avoid disclosure of improper acts or embarrassing information,” Levin said in a statement.

“But in reviewing the CIA-proposed redactions, I saw multiple instances where CIA proposes to redact information that has already been publicly disclosed in the Senate armed services committee report on detainee abuse that was reviewed by the administration and authorized for release in 2009.”

Mark Udall, a Colorado Democrat on the intelligence committee who has been among Brennan’s most aggressive critics, accused the CIA of trying to “face its past with a redaction pen, and the White House must not allow it to do so.”